Petroleum refining and petrochemical processes frequently involve isomerization processes. Isomerization processes are widely used by many refiners and processers to rearrange the molecular structure of hydrocarbons. In some instances it is more desirable to have more highly branched hydrocarbons. Generally, these more highly branched hydrocarbons have relatively high octane ratings. In other instances it is more desirable to have straight chain paraffins, for example for feed to a stream cracker or for production of chemicals. Accordingly, isomerization can involve conversion of iso-paraffins into normal paraffins or the conversion of normal paraffins into iso-paraffins. Additionally, due to equilibrium factors, various hydrocarbon feed streams are isomerized in different zones.
For example, C5 and C6 hydrocarbons streams are often isomerized in the same zone. When the isomerization comprises of converting iso-paraffins into normal paraffins, a large amount of C4 hydrocarbons may be formed as a product of cracking and disproportionation reactions.
The isomerization zone typically includes a stabilizer to separate the effluent from the isomerization zone. As a result of the high amount of light ends in the effluent, the stabilizer requires an LPG draw (C3/C4 hydrocarbons). However, since the LPG draw contains some amount of chloride, typically as HCl, the LPG draw cannot be directly routed to, for example, a saturated gas plant, as the LPG draw contains some amount of chlorides. Therefore, a stripping zone is used to remove the light ends (C2− hydrocarbons) and chlorides. Then, the LPG draw can be passed, along with light ends from other sources, to the saturated gas plant where the combined stream can be cleaned (sulfur removed) and fractionated. Any C4 hydrocarbons could be recovered and recycled back to be isomerized in a C4 hydrocarbons isomerization unit.
It is believed that a more effective and efficient process for recovering the C4 hydrocarbons would be desirable.